VMG Running in 18.3 to 18.7 kts
Post by iconPost by YourMomSA | 2018-01-03 | 16:57:09

Hi Cvetan -

This came up briefly in another thread, but I can't find it now. Zezo's VOR polars cross over from LG to HG at 18.8, whereas VR wants the change at 18.3.

Which is correct? If we're in 18.5, should we ignore VR's "wrong sail" notifications and their sail selection speed indicators, and trust that we're actually doing a better speed with LG? Or will our boat do the speed VR's sail selection screen says it will, such that we should switch to HG in the 18.3-18.7 range even though Zezo says LG?

I ask because we're coming up on an area where the router wants us to keep going until we hit 18.7, and then gybe away, having avoided the sailchange until the last moment. If the right time for the sailchange is 18.3, that strategy may be wrong. We'd need to either gybe an hour or more earlier, when we hit 18.3, or do two extra sailchanges (go to HG, gybe, and then later go back to LG).

I realize that when it's a temporary situation like this, it can be worthwhile to ignore the "wrong sail" notifications anyway. I just want to make sure I understand which sail is truly faster between 18.3 and 18.7 kts windspeed.


commenticon 3 Comments
Post by iconPost by zezo | 2018-01-03 | 17:17:37
VR crossover point is 18.5 exactly.
at 18.6 the speed difference is 0.12 kts (TWA 141)
at 18.8 is 0.2
at 19 around 0.3

So the balance point is around two hours @ 18.9 knots or so

I'm waiting for the new GRIB too ;-)
Post by iconPost by YourMomSA | 2018-01-03 | 17:51:28
Outstanding, thanks for the quick response. Great job in both active races. I'm very curious to see how fast your Spindrift gets to New Zealand.
Post by iconPost by zezo | 2018-01-03 | 18:08:49
Hmm the problem solved itself with the new GRIB ;-)

I have to rewrite that code - it uses legacy table in km/h wind speed because that was what VR used in the wind square times - integer km/h and degrees so the polar calculation was simple table lookup. Now with the interpolation it does not make sense anymore, but it's working, tested and debugged code.

The multi is doing good run. Still behind the record holder but catching up.

Edit: Ooops. It's not fixed, just shifted earlier. Looks about even for me with or without sail change.
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